This application is based upon and claims benefit of priority of Japanese Patent Application No. 2001-66290 filed on Mar. 9, 2001, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a method of manufacturing a fuel injector for supplying fuel to an internal combustion engine.
2. Description of Related Art
A fuel injector supplies fuel to an internal combustion engine in a controlled manner. As shown in FIG. 1, a valve body and an injection plate having fuel injection holes are installed at a bottom end of a cylindrical housing. The injection holes are opened or closed according to reciprocal movement of a valve needle which is slidably disposed in the cylindrical housing to be electromagnetically driven.
In a conventional manufacturing method, the valve body is connected to the bottom end of the cylindrical housing by welding after other components are assembled together with the cylindrical housing. Then, the injection plate is welded to the bottom surface of the valve body. The conventional method of assembling the valve body and the injection plate to the cylindrical housing is shown in FIGS. 7-10.
As shown in FIG. 7, a valve body 100 is forcibly inserted into a bottom end of a cylindrical housing 101. Then, the valve body 100 is connected to the cylindrical housing 101 by performing laser-welding around the outer periphery of the cylindrical housing 101, as shown in FIG. 8. Then, as shown in FIG. 9, the bottom end of the valve body 100 is forcibly inserted into a sidewall 104 of a cup-shaped injection plate 102. Then, as shown in FIG. 10, the injection plate 102 is connected to the valve body 100 by performing laser-welding around the outer periphery of the side wall 104.
The valve body 100 has a circular hole and a circular valve seat 103 formed around the circular hole in a tapered shape. The circular valve seat has to closely abut the bottom end of the valve needle to close the injection holes formed on the injection plate 102. However, there has been a problem, in the conventional method described above, that the circular valve seat 103 is distorted and its roundness is not maintained. If the roundness of the circular valve seat 103 is not kept, the circular valve seat 103 does not correctly abut the bottom end of the valve needle, and thereby fuel leaks through the valve seat portion at a time fuel supply has to be stopped. If the fuel supply is not shut at the required timing, an air-fuel ratio cannot be properly controlled and quality of exhaust gas is adversely affected.
The roundness of the circular valve seat 103 is damaged in the conventional process because the valve body 100 is forcibly inserted into the inner bore of the cylindrical housing 101 and then both the valve body 100 and the cylindrical housing 101 are connected together by welding. Further, the bottom end of the valve body 100 having the circular valve seat 103 is forcibly inserted into the sidewall 104 of the injection plate 102 and then both are connected together by welding. The roundness of the circular valve seat 103 is damaged by those forcible insertion and welding processes.
The present invention has been made in view of the above-mentioned problem, and an object of the present invention is to provide an improved method of manufacturing a fuel injector, in which a roundness of a circular valve seat is correctly kept.
A fuel injector comprises a cylindrical housing in which a valve needle is slidably disposed and a valve unit connected to a bottom end of the cylindrical housing. The valve unit is composed of a ring-shaped valve body having a circular valve seat on which the valve needle seats and a cup-shaped injection plate connected to a bottom end of the valve body. All components of the fuel injector other than the valve unit are assembled separately from the valve unit, and the valve unit is connected to the assembled fuel injector.
In a process of forming the valve unit, a bottom end of the ring-shaped valve body is coupled with a cup-shaped injection plate having a flat bottom plate and a circular sidewall. An inner diameter of the circular sidewall is so made that the valve body is coupled with the injection plate without deforming the circular valve seat formed on the valve body. Then, a bottom plate of the injection plate is connected to the bottom surface of the valve body by welding the bottom plate to the bottom surface of the valve body. Then, the valve body to which the injection plate is welded is inserted into the bottom end bore of the cylindrical housing. Then, the valve body is connected to the cylindrical housing by welding the outer periphery of the cylindrical housing.
A process for adjusting a shape and a roundness of the circular valve seat may be additionally performed, if such is needed, after the injection plated is welded to the valve body and before the valve body is connected to the cylindrical housing. In the adjusting process, a round rod having an end corner surface coinciding with the circular valve seat is forcibly pressed against the circular valve seat, and the round rod is rotated. In this manner, the surface of the circular valve seat is smoothed and its roundness is improved. The round rod adjusting the circular valve seat is easily inserted into an inner bore of the valve body because the valve body is not yet connected to the cylindrical housing at this stage.
According to the present invention, the roundness of the circular valve seat formed on the valve body is not damaged by the process of coupling and welding the injection plate to the valve body because the valve body is not forcibly inserted into the injection plate. Further, the circular valve seat is not deformed in the process of inserting and welding the valve body to the cylindrical housing because the valve body is reinforced by the injection plate at this stage. Therefore, the valve needle is correctly seated on the circular valve seat to close injection holes without causing fuel leakage therebetween.
Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiment described below with reference to the following drawings.